u n i ve r s i t y o f co pe n h ag e n

Element scavenging by recently formed travertine deposits in the alkaline springs fromthe Oman Semail Ophiolite

Olsson, Jonas; Stipp, Susan Louise Svane; Gislason, S.R.

Published in:Mineralogical Magazine

DOI:10.1180/minmag.2014.078.6.15

Publication date:2014

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Element scavenging by recently formed travertine deposits

in the alkaline springs from the Oman Semail Ophiolite

J. OLSSON1,2,*, S. L. S. STIPP1 AND S. R. GISLASON2

1 Nano-Science Center, Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100

København Ø, Denmark2 Nordic Volcanological Institute, Institute of Earth Sciences, University of Iceland, Sturlugata 7, 101 Reykjavik,

Iceland

[Received 6 May 2014; Accepted 9 December 2014; Associate Editor: T. Rinder]

ABSTRACT

Ultramafic rocks, such as the Semail Ophiolite in the Sultanate of Oman, are considered to be a

potential storage site for CO2. This type of rock is rich in divalent cations that can react with dissolved

CO2 and form carbonate minerals, which remain stable over geological periods of time. Dissolution of

the ophiolite mobilizes heavy metals, which can threaten the safety of surface and groundwater

supplies but secondary phases, such as iron oxides, clays and carbonate minerals, can take up

significant quantities of trace elements both in their structure and adsorbed on their surfaces.

Hyperalkaline spring waters issuing from the Semail Ophiolites can have pH as high as 12. This

water absorbs CO2 from air, forming carbonate mineral precipitates either as thin crusts on the surface

of placid water pools or bottom precipitates in turbulent waters. We investigated the composition of the

spring water and the precipitates to determine the extent of trace element uptake. We collected water

and travertine samples from two alkaline springs of the Semail Ophiolite. Twenty seven elements were

detected in the spring waters. The bulk of the precipitate was CaCO3 in aragonite, as needles, and

rhombohedral calcite crystals. Traces of dypingite (Mg5(CO3)4(OH)2·5H2O) and antigorite

((Mg,Fe)3Si2O5(OH)4) were also detected. The bulk precipitate contained rare earth elements and

toxic metals, such as As, Ba, Cd, Sr and Pb, which indicated scavenging by the carbonate minerals.

Boron and mercury were detected in the spring water but not in the carbonate phases. The results

provide confidence that many of the toxic metals released by ophiolite dissolution in an engineered

CO2 injection project would be taken up by secondary phases, minimizing risk to water quality.

KEYWORDS: travertine, ophiolite, CO2 storage site, heavy metals.

Introduction

CAPTURING and storing CO2 in rock as carbonate

minerals, formed during injection of CO2 into

geological formations, has great potential for

reducing atmospheric concentrations of anthro-

pogenic CO2. Some carbonate minerals form

readily in nature but by increasing our under-

standing, the processes could be enhanced and

used on an industrial scale. Improved under-

standing of carbonate mineral formation would

also contribute towards more robust models for

predicting the global carbon balance (Mackenzie

and Andersson, 2013). Field-scale projects, where

CO2-charged waters are injected into the subsur-

face, have already begun, e.g. into basaltic

formations in Iceland and northwestern USA

(Alfredsson et al., 2008; Oelkers et al., 2008;

* E-mail: jolsson@nano.ku.dkDOI: 10.1180/minmag.2014.078.6.15

Mineralogical Magazine, November 2014, Vol. 78(6), pp. 1479–1490

# 2014 The Mineralogical Society

OPEN ACCESS

This paper is published as part of a special issue inMineralogical Magazine, Vol. 78(6), 2014 entitled‘Mineral–fluid interactions: scaling, surface reactivityand natural systems’.

Gislason et al., 2010; McGrail et al., 2011;

Alfredsson et al., 2013; Gislason and Oelkers,

2014). Carbon dioxide-enriched waters are

corrosive (at 1 atm CO2, pH = 3.6) and when

injected into geological formations, they attack

the host rock, increasing weathering rates. Mafic

and ultramafic rock releases ions such as Ca, Mg

and Fe, which can combine with the dissolved

CO2 and form stable carbonate phases. The

overall reaction is energetically favourable

(Lackner et al., 1995) but depends on fluid

composition, ionic strength, reduction potential,

pH, temperature and total pressure (Stumm, 1992;

Krauskopf and Bird, 1994; Gislason et al., 2010;

Olsson et al., 2012). Mafic and ultramafic rocks

are well suited to CO2 storage because of their

large divalent cation:Si ratio, and thus greater

capacity for carbon capture, and lower tendency

to form clay minerals, which use up cations, thus

decreasing carbonation potential. Reaction in

these rock formations neutralizes the acidic

waters, thus promoting carbonate mineral preci-

pitation. However, injecting CO2-charged waters,

and the ensuing dissolution of the host rock, can

mobilize toxic heavy metals, such as As, Ba, Cd,

Cr, F, Hg and Pb (Turekian and Wedepohl, 1961),

which could pose a risk for surface and ground-

water supplies (Aiuppa et al., 2000; Flaathen et

al., 2009; Galeczka et al., 2013; Olsson et al.,

2013; Galeczka et al., 2014; Olsson et al., 2014).

The Semail Ophiolite of the Sultanate of Oman

is a potential site for CO2-charged water injection

for carbon sequestration (Kelemen and Matter,

2008; Matter and Kelemen, 2009; Paukert et al.,

2012). It is one of the largest and best preserved

ophiolites in the world and covers ~30,000 km2,

with an average thickness of ~5 km (Neal and

Stanger, 1984b; Nicolas et al., 2000; Chavagnac

et al., 2013a). About 30% of this volume is the

ultramafic rock, peridotite, which contains mostly

olivine and pyroxene and

As protons are released, pH decreases. The

minerals observed are calcite and aragonite (both

CaCO3), as well as brucite (Mg(OH)2) and

antigorite ((Mg,Fe)3Si2O5(OH)4) (Neal and

Stanger, 1984a; Chavagnac et al., 2013a).

Spring waters in natural systems can be

considered as an analogue for CO2 injection

into ophiolites in general (Kelemen and Matter,

2008; Beinlich et al., 2010, 2012; Paukert et al.,

2012) and by studying the uptake of trace metals

into the secondary phases, we can gain informa-

tion about their mobility as a result of CO2injection. The aim of this study was to analyse

ultramafic spring waters and the natural precipi-

tated products to investigate the extent of trace

element uptake into the solids and loss from the

solution phase.

Sampling and analyses

The samples were collected on 13 and 14 January

2011, a total of eight liquid and eight solid

samples from two alkaline springs in Oman. The

springs were located near the village of Falaji

(Spring #1) and Qafifah (Spring #2) discharging

from partly serpentinized peridotite and gabbroic

rocks. Samples from the two springs and the

surroundings have been described elsewhere

(Nicolas et al., 2000; Kelemen and Matter,

2008; Matter and Kelemen, 2009; Paukert et al.,

2012; Chavagnac et al., 2013a,b). pH and

alkalinity were measured in the field. The pH

was measured using a plastic body, double

junction electrode, connected to a CyberScan

310 pH meter, calibrated against certified buffers

(pH 4.01, 7.00, 10.01 and 12.46) from Oakton.

Electrode and buffers were at the same temp-

erature as the spring water. Alkalinity was

determined by Gran titration, using HCl (Gran,

1952; Stumm and Morgan, 1981). Samples for ion

chromatography (IC) and inductively coupled

plasma optical emission spectrometry (ICP-OES)

were filtered in the field through 0.2 mm celluloseacetate membranes. They were collected in plastic

vials that had been flushed three times with the

sample. Samples for ICP-OES were acidified with

concentrated, suprapure HNO3 (1:100) onsite to

prevent precipitation of solids during transport.

An overview of the fluid samples and locations is

found in Table 1.

Solid samples were collected in plastic bags

and freeze dried for 24 h in the laboratory to

minimize alteration. The solids were character-

ized by means of standard X-ray powder

diffraction (XRD) and scanning electron micro-

scopy (SEM), using instrument conditions and

procedures described by Olsson et al. (2014).

Residual water in the pores could not be removed

by freeze drying and is responsible for precipita-

tion of halite (NaCl) observed in the XRD

patterns. The solid samples were flash dissolved

in a 1% HNO3 solution which dissolves the

carbonate mineral phases while minimizing the

dissolution of possible silicate mineral grains

TABLE 1. Summary of the sample data. At each location, a sample of travertine and spring water werecollected. Samples OM01�05 were collected on 13 January 2011 in Spring #1 and Samples OM06, 07 and08 were collected on 14 January 2011 in Spring #2.

Sample set Distance from spring — Location — Water temperature pH Total alkalinity(m) Latitude Longitude (ºC) (meq/l)

Spring #1OM01 0 22.8378 58.0568 30.6 11.6 5.9OM02 5 � 3 22.8379 58.0568 29.6 11.6 5.9OM03 10 � 3 22.8379 58.0567 26.8 11.7 6.3*OM04 13 � 3 22.8379 58.0567 25.3 11.6 5.8OM05 18 � 3 22.8380 58.0567 24.0 11.6 5.9

Spring #2OM06 0 22.9045 58.4245 20.3 11.5 3.2OM07 15 � 3 22.9044 58.4246 22.3 10.8 2.2OM08 30 � 3 22.9042 58.4249 20.3 10.9 1.9

* No filtration

ELEMENT SCAVENGING BY RECENTLY FORMED TRAVERTINE

1481

originating from the ophiolite. Selected spring

water and flash-dissolved samples were analysed

at Analytica-SGAB, Luleå, Sweden, using ICP-

OES and inductively coupled plasma sector field

mass spectroscopy (ICP-SFMS). Mercury was

measured with atomic fluorescence spectroscopy

(AFS). The elements measured by IC were Cl�,F� and SO4

2� and by ICP-OES were Al, As, B,Ba, Br, Ca, Cl, Cr, Fe, K, Li, Na, Mg, Mn, Mo, P,

S, Sb, Si, Sr, Ti, V and W. In-house, multi-

element standards were used for calibration and

these are regularly tested against the Standard

Canadian River Waters (SLRS-4) and single

element international standards (SPEX). The

uncertainties of the IC and ICP-OES data were

estimated from analytical measurements. The

reproducibility of duplicate samples for both

methods is within 5%.

The geochemical speciation program

PHREEQC (Parkhurst and Appelo, 1999), with

the MINTEQ database (Allison et al., 1991), was

used to determine the charge balance of the spring

water samples, to estimate the dissolved inorganic

carbon (DIC) and the saturation state of selected

solid phases. The saturation state of lansfordite

(MgCO3·5H2O) was determined in a parallel

PHREEQC operation run using the LLNL

database (Wolery, 1992). The charge balance of

all of the spring water samples was within 2% and

ionic strength was

TABLE3.Theelem

entconcentrationofthesolidtravertineandthespringwatersfrom

sites.Thetravertineconcentrationswerenorm

alized

toonemole

of

CaC

O3.Smallam

ounts

ofother

elem

ents

weredetectedin

thetravertine,

includingCe,

Dy,Er,Eu,Gd,Ho,La,

Nd,Pr,Sm,Tb,Tm

andYb.

Ca

Al

Co

Fe

KMg

Na

PSi

Sr

Ti

VZn

Springwaters(unit:m

M)

#1

2.15

1.66

10�4

1610�7

4.7610�5

0.255

1.72610�3

9.5

7.26

10�5

0.00407

0.00115

5610�7

1.46

10�6

4.8E-05

#2

0.0314

8.66

10�6

3610�7

BDL

0.0814

1.52

3.8

BDL

0.163

1.46

10�5

BDL

3.06

10�6

BDL

Solidtravertine(unit:mmol/molofCaC

O3)

#1

–0.0354

2.5610�4

1.786

10�2

0.286

0.964

26

0.0218

0.146

0.495

1.9610�4

1.76

10�4

6.11610�3

#2

–0.0157

2.6610�4

1.046

10�2

BDL

11.3

5.8

0.0128

0.850

2.26

1.9610�5

2.86

10�5

8.85610�3

BDL,below

detectionlimit.Thedetectionlimitsare(m

M):Ce,

46

10�8;Dy,36

10�8;Er,3610�8;Eu,3610�8;Fe,

76

10�6;Gd,36

10�8;Ho,36

10�8;

La,

46

10�8;Lu,3610�8;Nd,3610�8;P,2610�5;Pr,4610�8;Sm,3610�8;Tb,36

10�8;Th,9610�8;Ti,26

10�8;Tm,2610�8;andYb,3610�8.

TABLE2.Theelem

entconcentrationofthesolidtravertineandthespringwatersfrom

thetwosites.Thetravertineconcentrationswerenorm

alized

toonemole

of

CaC

O3.Concentrationoftheelem

entsin

thetravertineandspringwater

whichpotentially

pose

athreat

tothequalityofgroundwater

supplies

areincluded

andthe

WHO

standardfordrinkingwater

isshownforcomparison(W

HO,2008).Allconcentrationsin

thespringwatersarebelow

therecommendlimitsofWHO.The

precipitatingtravertinescavenges

alltoxic

elem

entsexceptHg.

Spring

As

BBa

Cd

Cr

Cu

FHg

Mn

Mo

Ni

Pb

Springwaters(unit:nM)

#1

BDL

BDL

19.2

BDL

BDL

11

5.896

10�3

0.232

3.68

BDL

7.99

0.196

#2

BDL

4700

0.442

BDL

0.819

BDL

1.916

10�3

4.846

10�2

0.881

10.1

9.46

8.066

10�2

WHO

threshold

1.36

102

4.66

104

5.16103

27

9.66

102

3.16

104

7.96104

30

7.36

103

7.36102

1.26103

48

Solidtravertine(unit:mmol/molofCaC

O3)

#1

7.7610�5

BDL

8.78610�3

3.16

10�6

2.1610�4

3.76

10�4

NA

BDL

9.51610�3

BDL

2.616

10�3

6.9610�5

#2

4.8610�5

BDL

4.14610�2

4.06

10�6

2.1610�4

2.76

10�4

NA

BDL

1.77610�2

8.66

10�6

1.336

10�3

9.6610�6

BDL,below

detectionlimit.Thedetectionlimitsare(m

M):As,4610�6;B,96

10�4;Cd,2610�8;Cr,2610�7;andMo,5610�7.

FIG. 3. Element concentrations measured as a function of distance downstream in Spring #2, including the

concentration of (a) the alkali metals, alkali earth metals and Si, and (b) anions. The dissolved inorganic carbon was

estimated using PHREEQC. Uncertainties are within the dimension of the symbols.

FIG. 2. Element concentrations in the stream water measured with distance downstream in Spring #1, including the

concentration of (a) the alkali and alkali earth metals, (b) transition metals, (c) other metals of interest, and

(d) anions. The dissolved inorganic carbon was estimated using PHREEQC. Most concentrations remain constant

with distance downstream except for Mg, Fe, Ni and Zn. Uncertainty is generally within the dimension of the

symbols. No uncertainty limits are available for Ti.

1484

J. OLSSON ET AL.

The travertine deposits

The travertine deposits in the alkaline springs were

analysed using XRD and SEM/EDXS to identify

the mineral composition and the morphology of the

samples collected. The precipitates of the springs

consist mainly of aragonite and calcite. The XRD

patterns of the samples from Spring #1 also showed

traces of dypingite (Mg5(CO3)4(OH)2·5H2O), anti-

gorite ((Mg,Fe)3Si2O5(OH)4) and halite (NaCl). We

were not able to detect amorphous material. With

the exception of halite, these precipitates are typical

of the hyperalkaline springs of the Oman ophiolites

(Neal and Stanger, 1984a; Chavagnac et al.,

2013a). The presence of halite can be explained

by evaporation of residual water when the samples

were freeze dried. The SEM images of the thin-

surface precipitate show rhombohedral calcite in a

network of aragonite crystals (Figs 1a, 4a), whereas

the precipitate from the bottom of Spring #1

consisted of dypingite rosettes with bouquets of

aragonite (Fig. 4b). The XRD patterns showed that

the travertine samples from Spring #2 were

predominantly of aragonite. The SEM images

showed small networks of rosettes growing on

aragonite needles (Fig. 4c,d). The rosettes were rich

in magnesium, indicating either dypingite or brucite

(Power et al., 2007; Chavagnac et al., 2013a).

The chemical composition of the carbonate

minerals in the travertine samples was quantified

by flash dissolving the samples and analysing the

chemical components released to solution. The

average composition for travertine from both

springs was determined and normalized to one

mole of dissolved CaCO3, assuming all Ca ions

originated from pure CaCO3 (Table 2 and 3). The

travertine contains a wide range of elements,

including rare earth elements (REE) and the toxic

elements: As, Ba, Cd, Cr, Cu, Mn, Mo, Ni and Pb.

Some elements were below detection limits in the

spring water but were found in the travertine

which indicates that these trace elements have, to

a significant extent, been taken up by the

precipitates. Elements observed in the spring

waters were also found in the travertine, except

for B and Hg. Thus, most toxic elements released

from an engineered CO2 injection project would

be sequestered by secondary minerals but some,

namely B and Hg, would not.

Mineral saturation estimated using PHREEQC

Numerous carbonate minerals can form in the

alkaline spring waters of Oman. We investigated

which phases are supersaturated and evaluated if

they would be likely to precipitate while the spring

water equilibrated with atmospheric CO2.

Knowing which phases are present in the travertine

is important because trace element coprecipitation

and adsorption are controlled by the composition

and structure of the sorbing phase. We used the

PHREEQC geochemical speciation model to

determine the saturation states. In the model,

CO2 was added to the spring water in small steps

and the saturation states were calculated until

equilibrium with atmospheric CO2 was reached

(SI = �3.4). No minerals were allowed toprecipitate. For the model, we used data from

Sample OM01 (water from Spring #1) and OM06

(from Spring #2), including temperature, chemical

composition, pH and alkalinity. The saturation

index (SI) maximum at pH 10.6, for the pure

carbonate phases, corresponds to the point at which

the CO32� concentration peaks. At lower pH, the

HCO3� ion dominates.

PHREEQC predicted that the supersaturated

phases in Spring #1 would be calcite (hexagonal

CaCO3), aragonite (orthorhombic CaCO3) and

dolomite (CaMg(CO3)2) and in Spring #2, artinite

(Mg2(CO3)(OH)2·3H2O), dolomite, huntite

(Mg3Ca(CO3)4), magnesite (MgCO3), hydromag-

nesite (Mg5(CO3)4(OH)2·4H2O), calcite and

aragonite (Fig. 5). Lansfordite (MgCO3·5H2O,

not shown) and nesquehonite (MgCO3·3H2O)

were predicted to be undersaturated. Inorganic

synthesis of dolomite, magnesite and huntite at

atmospheric pressure and temperature below 40ºC

has never been reported (Deelman, 1999; Saldi et

al., 2009; Deelman, 2011; dos Anjos et al., 2011)

so they are not expected to precipitate in the

Oman spring waters. Hydrous magnesium carbo-

nate phases, such as artinite, hydromagnesite and

dypingite (Mg5(CO3)4(OH)2·5H2O), are far more

likely to form (Hsu, 1967; Botha and Strydom,

2001; Hopkinson et al., 2008; Hänchen et al.,

2008). No thermodynamic data are available for

predicting the saturation state of dypingite but

precipitation of both dypingite and hydromagne-

site can be mediated by microorganisms (Ming

and Franklin, 1985; Braithwaite and Zedef, 1996;

Power et al., 2007; Cheng and Li, 2009; Power et

al., 2009). The hydrous magnesium carbonate

minerals are relatively stable and form more

readily than magnesite at temperatures of

low as a result of serpentinization. In an

engineered CO2 injection scenario, serpentiniza-

tion is expected to be minimal and therefore the

relative concentration of Mg would be much

higher, i.e. ~50 times higher than the Ca

concentration (Boudier and Coleman, 1981). In

this scenario, magnesium phases would probably

dominate the secondary phases and these could

complement the scavenging of contaminants

above that observed in this study. More research

is needed to confirm this assessment.

Conclusions and outlook

The spring waters from the Semail Ophiolite are

highly alkaline, with pH between 11.6 and 10.9.

FIG. 4. SEM images of the precipitates from Spring #1 (a,b) and Spring #2 (c,d). (a) The carbonate surface film from

Fig. 1a, sample OM01: rhombohedral calcite in a network of aragonite needles. (b) Sample OM03, dypingite

rosettes (black arrow) surrounded by acicular aragonite; (c) Sample OM08 from Fig. 1b, crystals of aragonite;

(d) close-up of the same area showing a secondary phase (black arrows) growing from or nucleating on the aragonite

crystals. These crystals are probably dypingite. Samples OM03 and OM08 were collected from the bottom of the

springs.

1486

J. OLSSON ET AL.

The most prominent cations are Ca, K, Na, Mg

and Si; anions are Br, Cl, F and SO4. Within

uncertainty, the concentrations remain constant

downstream for Spring #1 but complicated

changes in the chemical composition for Spring

#2 suggest dissolution and precipitation reactions

and probably input from a second spring.

Travertine samples from the alkaline springs

consist of aragonite often mixed with calcite and

traces of dypingite and antigorite. Other phases

that were supersaturated in the spring waters, and

predicted to precipitate at atmospheric conditions,

were artinite and hydromagnesite. The most

common alkali, alkali earth and transition metals

were detected in the solid travertine samples

along with REE and toxic elements such as As,

Ba, Cd, Cr, Cu, Mn, Mo, Ni and Pb. This suggests

active and effective scavenging by precipitation

of the carbonate minerals, thus limited mobility of

contaminants. However, boron and mercury were

not removed from the water. If they were present

at increased concentrations they would pose a

health risk.

Our study shows that several carbonate

minerals precipitate readily in groundwater from

the Semail Ophiolite when CO2 dissolves in it.

Aragonite and calcite dominate the precipitates

and these phases are well known to incorporate

trace elements. Thus we can conclude that toxic

metals that are released during dissolution of the

ophiolite are taken up by the travertine. From this

we can draw analogies with likely scenarios for

geological CO2 sequestration. Although the

reaction conditions in an engineered CO2 injec-

tion system would be different from the condi-

tions in our study, in both cases, Ca, Mg, Fe, CO2and trace metals would be present and where

carbonate minerals such as calcite reach super-

saturation and precipitate, we would expect the

precipitates to scavenge the trace elements. Thus

we can conclude that if toxic trace metals are

released by dissolution of mafic or ultramafic

rocks during CO2 injection, they would probably

be sequestered by the carbonate minerals or other

secondary phases (such as Fe oxides and clays)

that form as the waters approach equilibrium.

Thus, this ophiolite system serves as a natural

analogue, providing insight into what could be

expected from engineered injection of CO2 into

porous peridotite and offers an example scenario of

what might happen if CO2 leaks from an injection

site. Such studies of natural analogues are useful

for providing information about natural processes

that laboratory studies of ideal systems cannot.

Studying natural systems gives information about

how elements or products are favoured or

disfavoured in complicated competitive reactions.

Acknowledgements

The authors thank Eydı́s Salome Eirı́ksdóttir,

Helene Almind and Iwona Gałeczka for technical

help. For assistance in the field and transport of

the samples, Pablo Garcı́a del Real and Amelia

FIG. 5. The saturation state of selected carbonate minerals as a function of pH (addition of CO2) with respect to the

water from (a) Spring #1 at Sampling site OM01, and (b) Spring #2 at Sampling site OM06. Secondary minerals

were not allowed to precipitate in the model. The reaction progress is from right to left in the figures. Before the

addition of CO2, the pH was 11.6 and temperature was 30.6ºC for Spring #1, and 11.5 and 20.3ºC for Spring #2. The

logarithm of the initial partial pressure of CO2 was �9.3 bar for Spring #1 water and �8.3 bar in Spring #2. Thelogarithm of the final CO2 pressure after the solution reached equilibrium with the Earth’s atmosphere was �3.4 bar

or 395 ppm CO2.

ELEMENT SCAVENGING BY RECENTLY FORMED TRAVERTINE

1487

Paukert are acknowledged. The authors are also

grateful to Christophe Monnin and two anon-

ymous reviewers for their comments during

review. The work was funded by the Nordic

Volcanological Institute (NORDVULK), the

Institute of Earth Sciences, Reykjavı́k, Iceland;

the NanoGeoScience Group, Nano-Science

Center, Department of Chemistry, Copenhagen,

Denmark; a travel grant for the IODP/ICDP

Workshop on ‘‘Geological carbon capture &storage in mafic and ultramafic rocks’’ sponsoredby the ESF Magellan Workshop Series; and by

the European Commission Framework 7, through

the CarbFix Project, Grant Agreement No: FP7

283148.

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